How do I remap colors in Cycles?
It looks like what you are looking for its the color ramp node.


The color ramp node can take any value between 0 and 1 (values outside this range will be clamped to 0 or 1) in it's fac input socket.  And it will output the respective color from the gradient specified by the color swatches just like in BI.

Use the + and - buttons to add more swatches.
Use the &lt;-&gt; button to flip the order of swatches.
The color (and alpha value) of the selected swatch is set with the large color strip at the bottom of the node.
You can adjust the exact position of the selected swatch with the pos spinner.
The alpha gradient is outputted through the alpha output socket.


So it could, as you mentioned, be used to adjust the affect of an image texture on a mix shader (the image texture could just get plugged directly into the mix shader if no color adjustment is needed though).